1. Field of the Invention
The present invention relates to an acoustic apparatus comprising a Helmholtz resonator and, more particularly, to a structure of an acoustic cabinet with a resonance port constituting the Helmholtz resonator.
2. Description of the Prior Art
As an acoustic apparatus utilizing Helmholtz resonance, a phase-inversion type (bass-reflex type) speaker system is known. FIGS. 5A and 5B are respectively a perspective view and a sectional view showing an arrangement of a bass-reflex speaker system. In the speaker system shown in FIGS. 5A and 5B, a hole is formed in the front surface of a cabinet 21, and a vibrator (dynamic speaker unit) 24 comprising a diaphragm 22 and a transducer 23 is mounted over the hole. An open duct port 26 having a sound path 25 is arranged below the vibrator 24. In the bass-reflex speaker system according to the standard design, a resonance frequency (antiresonance frequency) f.sub.op caused by an air spring of the cabinet 21 and an air mass in the sound path 25 is set to be lower than the lowest resonance frequency f.sub.or of the vibrator 24 which is mounted on the bass-reflex cabinet or, in some cases, a lowest resonance frequency f.sub.o inherent in the vibrator. At a frequency higher than the antiresonance frequency f.sub.o, the phase of a sound pressure from the rear surface of the diaphragm 22 is reversed at the sound path 25. In front of the cabinet 21, a direct radiation sound from the front surface of the diaphragm 22 and a sound from a port opening portion 27 are consecutively in phase with each other and are in-phase added to each other, thus increasing a sound pressure. More specifically, the lowest resonance frequency of the system can be expanded to the antiresonance frequency f.sub.op of the resonator by this in-phase addition. As a result, according to the optimally designed bass-reflex type speaker system, frequency characteristics of an output sound pressure can be expanded below the resonance frequency f.sub.oc or f.sub.o of the vibrator 24. As indicated by an alternate long and two short dashed curve in FIG. 6, a uniform reproduction range can be widened as compared to that of an infinite plane baffle or a closed baffle.
The conventional speaker cabinet structure in the bass-reflex acoustic apparatus employs a structure in which the prismatic or cylindrical resonance port 26 is mounted on the front surface of the cabinet 21, as shown in FIG. 5B, or a structure in which the lower surface of the cabinet 21 is doubled, and a space between these lower surfaces is used as the sound path 25 to constitute the resonance port 26, as shown in FIG. 7.
However, in the conventional speaker cabinet structure, the length of the resonance port is limited depending on the size of the cabinet, and a predetermined resonance frequency cannot always be obtained. The baffle area of the opening surface of the resonance port is increased, and the speaker cabinet becomes large in size.